HERO, you have rightly articulated the reasons behind students' failure in English.I will add some more
1. English is not our native language.To have good grip over a foreign language is naturally difficult. What we need to do is practice.Lack of practice makes the style monotonous.As writing is most difficult task in preparation thats why students, usually,dont pay much heed towards this ordeal.
2.Secondly,students usually make the mistake of jotting down common sentences heard or read fron news paper or elsewhere.This is equally destructive.We are supposed to show our grip over the language.For example while writing Global warming most students attempt the essay in such a way as they were attempting everyday science.
3.Lack of knowledge,solid arguments,qoutes,sayings.
4.Most students take essay easy as they think its not difficult to ink more or less 25 pages.Such kind of thinking makes them relaxed about English.Can you believe a student having 713 marks was failed in English
5.Lack of analytical skills.Sometimes students dont know how to summarize something.Examiner doesnt have enough time to read one argument stretched over pages.We need to be brief yet illustrative.
6.Spellings and grammatical mistakes challenge the patience of checker.We need to avoid them at all.
7.Same are the mistakes which are usually made in the paper of composition.Insufficient vocabulary leads to destruction.
This was my personal assessment.....................every person has his own angle to see the things